Field of the Invention
The present invention relates to a magnetic carrier, a two-component developer and a replenishing developer used in an image forming method for visualizing electrostatic images by an electrophotographic method, and an image forming method using the same.
Description of the Related Art
In known electrophotographic image forming methods, in general, an electrostatic latent image is formed on an electrostatic latent image bearing member by various techniques, and the electrostatic latent image is developed by applying a toner to the electrostatic latent image. For such development, two-component development is widely adopted, in which the toner is triboelectrically charged to an appropriate positive or negative charge level with carrier particles called magnetic carrier mixed with the toner. The charge applied to the toner acts as driving force of the development.
In two-component development, the magnetic carrier functions to stir, carry, and charge the developer, and thus, the functions of the carrier are clearly divided from those of the toner. Accordingly, the properties of the developer can be easily controlled.
With recent technological evolution in the field of electrophotography, difficult demands to image forming apparatuses are increased for high speed, long life, high definition, and high image quality. To respond to these demands, it is desired to enhance the performance of the magnetic carrier.
For example, Japanese Patent Laid-Open No. 4-93954 discloses an approach to reducing variation in density, or variation in color tinge for full color images, even in long-time use. The carrier used in this discloser features the core material thereof whose particles have asperities exposed at the surfaces even though the particles are coated with a resin. This approach helps to improve the performance of the magnetic carrier to some extent. Under the recent environment requiring high-speed copying, however, much load is placed on the toner due to the high specific gravity of the magnetic carrier particles, and thus, the life of the developer is shortened. Further improvement is therefore required in image quality and adaptability to environmental changes.
Accordingly, Japanese Patent Laid-Open Nos. 2006-337579, 2009-175666, and 2004-77568 disclose approaches, each using porous magnetic cores having pores therein to reduce the specific gravity. The use of these porous magnetic cores for magnetic carriers increases the lifetime of the developer, but creates new issues, such as leakage, blank dots, and graininess. It is imperative to develop a magnetic carrier and a two-component developer that can solve the new issues, and an image forming method using the carrier and developer.